The subject invention relates generally to photographic cameras of the reflex variety and, more particularly, to such a camera wherein an image formed therewithin, by light from an entrance pupil, may be viewed at a predetermined remote exit.
It is well known that various types of optical apparatus have been employed in prior art systems for precisely imaging light emanating from a point source, or appearing to so emanate, to a predetermined remote point. Such devices have found utility in various photographic applications and have additionally found application in spectroscopic and spectrometric devices.
It has long been known that a concave spherical-shape mirror may be utilized to reflect light from a point source to a remote predetermined point. In order to effect such reflection, it is often necessary that the point source be positioned off the optical axis of the spherical-shape mirror. The case of obtaining and/or fabricating such a spherical-shape mirror and, hence, the desirability of its use for the purposes aforesaid should be readily apparent.
Not infrequently, however, it is necessary that the optical means utilized for imaging a real or apparent point source to another point be essentially flat or planar. Where such is the case, it has been found expedient to resort to a mirror utilizing the principles of a Fresnel lens. Such a mirror may be envisioned by considering a concave spherical-shape mirror divided into a plurality of frusta, each formed by the intersection of a pair of spaced parallel planes with said mirror normal to the optical axis thereof. If each of said plural frusta are concentrically arranged on a flat surface, a reflective device will be created approximating the same optical qualities as said spherical-shape mirror, but one which is essentially planar. Such a planar approximation of a concave spherical-shape mirror may be referred to as a Fresnel-type mirror.
It should be readily apparent that the Fresnel-type mirror is preferred to its concave spherical-shape counterpart for many applications, if for no other reason, than because of its compactness of size and ease of fabrication.
Unfortunately, the off-axis use of a concave spherical-shape mirror, or its Fresnel-type counterpart, entails several disadvantages. As is well known in the art, the use of such structure invariably results in attendant image distortion, due to the inherent aberrations in the mirror's structure, such as astigmatism and coma.
Coma, of course, is an aberration affecting only points off the optical axis and it is known that coma is directly proportional to the distance of such points from said optical axis. It is known, too, that astigmatism varies with the square of the image height and, hence, we find that the very use to which it is desired to put the concave spherical-shape mirror, or its Fresnel-type counterpart (i.e., reflection in an off-axis mode), is one wherein significant coma and astigmatism will be introduced.
In order to provide point-to-point reflection, as aforesaid, without attendant astigmatism and coma, a novel image-reflection means was developed which is the subject of U.S. application Ser. No. 83,030, filed Oct. 22, 1970, for "Reflective Imaging Apparatus" by Nathan Gold, now U.S. Pat. No. 3,690,240, issued on Sept. 12, 1972 and assigned to the assignee of this application. The novel reflective imaging apparatus, aforesaid, envisions a relatively planar reflecting surface comprising a plurality of discrete facets each so oriented that light emanating from a real or apparent point source will be reflected therefrom and imaged at a remote predetermined point.
It has been found that in some applications requiring extremely high optical acuity, the use of a multi-faceted planar reflecting surface such as taught in U.S. application Ser. No. 83,030, aforesaid, does not provide an economic expedient.